Adding a preset amount of gain with just a single control, the Micro Amp is a great way to boost your signal for lead work or adjust between two different guitars with unmatched output (i.e. humbuckers to single-coils). It can also provide a permanent boost in a long effects chain where signal drop off is a problem.
A booster pedal to give you a little more oomph needed to overdrive your amp or boost your tone for a solo. The MXR Microamp can also compensate for differences in output between guitars without having to fiddle with amps settings, or simply act as an always-on booster that supplements your amp's tone.